UNIVERSITY  OF   CALIFORNIA   PUBLICATIONS 

COLLEGE  OF  AGRICULTURE 

AGRICULTURAL  EXPERIMENT  STATION 

BERKELEY,  CALIFORNIA 


THE  JUNE  DROP  OF 
WASHINGTON   NAVEL  ORANGES 

A  PROGRESS  REPORT 


BY 

J.  ELIOT  COIT  and  ROBERT  W.  HODGSON 


BULLETIN  No.  290 

January,  1918 


UNIVERSITY  OF  CALIFORNIA  PRESS 

BERKELEY 

1917 


Benjamin  Ide  Wheeler,  President  of  the  University. 

EXPERIMENT  STATION  STAFF 

HEADS  OF  DIVISIONS 

Thomas  Forsyth  Hunt,  Director. 

Edward  J.  Wickson,  Horticulture  (Emeritus). 

Herbert  J.  Webber,  Director  Citrus  Experiment  Station;  Plant  Breeding. 

Hubert  E.  Van  Norman,  Vice-Director;  Dairy  Management. 

William  A.  Setchell,  Botany. 

Myer  E.  Jaffa,  Nutrition. 
*  Robert  H.  Loughridge,  Soil  Chemistry  and  Physics   (Emeritus). 

Charles  W.  Woodworth,  Entomology. 

Ralph  E.  Smith,  Plant  Pathology. 

J.  Eliot  Coit,  Citriculture. 

John  W.  Gilmore,  Agronomy. 

Charles  F.  Shaw,  Soil  Technology. 

John  W.  Gregg,  Landscape  Gardening  and  Floriculture. 

Frederic  T.  Bioletti,  Viticulture  and  Enology. 

Warren  T.  Clarke,  Agricultural  Extension. 

John  S.  Burd,  Agricultural  Chemistry. 

Charles  B.  Lipman,  Soil  Chemistry  and  Bacteriology. 

Clarence  M.  Haring,  Veterinary  Science  and  Bacteriology. 

Ernest  B.  Babcock,  Genetics. 

Gordon  H.  True,  Animal  Husbandry. 

James  T.  Barrett,  Plant  Pathology. 

Fritz  W.  Woll,  Animal  Nutrition. 

Walter  Mulford,  Forestry. 

W.  P.  Kelley,  Agricultural  Chemistry. 

H.  J.  Quayle,  Entomology. 

D.  T.  Mason,  Forestry. 

J.  B.  Davidson,  Agricultural  Engineering. 

Elwood  Mead,  Rural  Institutions. 

H.  S.  Reed,  Plant  Physiology. 

W.  L.  Howard,  Pomology. 
|Frank  Adams,  Irrigation  Investigations. 

C.  L.  Roadhouse,  Dairy  Industry. 

O.  J.  Kern,  Agricultural  Education. 

John  E.  Dougherty,  Poultry  Husbandry. 

S.  S.  Rogers,  Olericulture. 

David  N.  Morgan,  Assistant  to  the  Director. 

Mrs.  D.  L.  Bunnell,  Librarian. 

Division  of  Citriculture 

J.  Eliot  Coit  Ira  J.  Condtt 

Robert  W.  Hodgson 


*  Died  July  1,  1917. 

t  In  co-operation  with  office  of  Public  Roads  and  Rural   Engineering,   U.    S. 
Department  of  Agriculture. 


THE  JUNE  DROP  OF  WASHINGTON  NAVEL  ORANGES 

A  Progress  Beporti 

BY 

J.  ELIOT  COIT  and  ROBERT  W.  HODGSON 


INTRODUCTION 

The  Washington  Navel  variety  when  grown  in  the  interior  valleys 
of  California  and  in  Arizona  has  always  been  subjected  to  large  annual 
diminution  of  crop  by  the  excessive  shedding  of  the  young  fruits. 
This  premature  fall  has  come  to  be  known  popularly  as  the  "June 
drop,"  although  it  is  by  no  means  confined  to  the  month  of  June  but 
may  occur  at  any  time  from  petal  fall  in  April  until  the  fruits  reach 
maturity.  Ordinarily  very  little  dropping  occurs  after  the  fruits 
reach  a  diameter  of  two  inches. 

Conservative  estimates  based  on  the  most  reliable  data  obtainable 
place  the  annual  loss  at  from  $1,225,000  to  $1,750,000.  In  many 
localities  in  the  interior  valleys  where  land  and  water  are  relatively 
cheap,  soil  conditions  ideal,  pests  few,  and  climatic  conditions  favor- 
able to  the  production  of  early  maturing  fruit  of  excellent  quality, 
the  light  crops  due  to  this  excessive  shedding  are  both  disconcerting 
and  unfortunate. 

For  a  number  of  years  requests  for  an  investigation  of  this  trouble 
have  been  sent  to  the  Experiment  Station  and  at  the  Davis  meeting 
of  the  State  Fruit  Growers'  Convention  a  resolution  was  passed 
calling  the  attention  of  the  University  authorities  to  the  urgent  need 
for  such  an  investigation.  In  compliance  with  this  request,  the  Divi- 
sion of  Citriculture  undertook  a  study  of  the  June  drop  phenomenon 
in  the  spring  of  1916,  and  has  since  accumulated  a  considerable  mass 
of  data.  While  the  investigation  is  by  no  means  complete  and  no 
simple  and  satisfactory  method  of  prevention  has  yet  been  worked 
out,  the  cause  of  the  trouble  and  the  nature  of  the  phenomena  involved 
seem  to  have  been  uncovered. 

The  fact  that  a  fungus  has  been  found  connected  with  certain 
phases  of  the  drop  phenomenon  has  led  some  growers  to  anticipate 


1  A  preliminary  technical  report  containing  pertinent  data  appeared  in  the 
Agricultural  Science  Series  of  the  University  of  California  Publications,  vol.  3, 
no.  3,  September  29,  1917.  "Some  Abnormal  Water  Relations  of  Citrus  Trees  in 
the  Arid  Southwest  and  Their  Possible  Significance, ' '  by  Robert  W.  Hodgson. 
A  more  complete  joint-authorship  technical  paper  is  in  course  of  preparation. 


204  UNIVERSITY    OF    CALIFORNIA EXPERIMENT    STATION 

that  spraying  with  fungicides  might  prove  an  effective  means  of 
control.  Unfortunately  neither  winter  spraying  nor  spraying  in  the 
bloom  has  proved  effective.  Inasmuch  as  our  investigations  have 
discovered  the  reasons  for  the  failure  of  sprays  and  have  indicated 
in  a  general  way  the  lines  along  which  efforts  for  prevention  or  control 
must  be  directed,  a  report  seems  warranted  at  this  time. 

The  shedding  of  the  young  navel  oranges  has  been  ascribed  to  vari- 
ous causes  among  which  are  irregular  or  deficient  irrigation,  cold 
irrigation  water,  irregular  fertilization,  injury  due  to  thrips,  fungus 
parasites,  and  climatic  conditions.  A  somewhat  comprehensive  pre- 
liminary study  of  the  situation  with  extended  observations  in  the 
field  very  soon  eliminated  all  but  the  last  two  possibilities.  Therefore, 
attention  was  early  directed  toward  attempts  to  isolate  fungi  from 
the  stigmas  of  the  young  flowers  and  from  the  navel  ends  of  the 
small  fruits. 

ALTERNARIA    CITRI    AND    ITS    CONNECTION    WITH    THE    JUNE    DROP 

For  many  years  growers  of  Washington  Navel  oranges  have  ex- 
perienced losses  from  a  black-rot  disease  of  the  fruit  which  usually 
is  first  observed  in  the  interior  of  the  navel  end  and  gradually  extends 
inward  toward  the  center.  The  infection  while  not  extensive  occasions 
annual  losses  varying  from  three  to  seven  per  cent,  This  disease  was 
noted  by  Newton  B.  Pierce  in  1892  and  was  first  described  by  him  in 
1902  as  "Black  Rot  of  the  Navel  Orange"  caused  by  Alternaria  citri. 

The  fruit  is  infected  when  quite  small,  probably  just  before  or 
soon  after  the  style  is  shed,  through  the  cracks  and  imperfections  in 
the  proliferations  of  the  navel.  The  fungus  is  a  weak  parasite  and 
remains  quiescent,  or  nearly  so,  during  the  growing  period  of  the 
young  fruit,  at  which  time  the  fruit  is  more  or  less  resistant  to  the 
encroachments  of  parasites.  "With  the  decline  in  vigor  incident  to 
approaching  maturity  the  fungus  becomes  active  and  exerts  a  stimu- 
lating influence  on  the  fruit,  causing  it  to  take  on  a  deep  reddish 
yellow  color  and  ripen  earlier  than  the  normal  fruit.  In  a  small  and 
restricted  area  the  cells  of  the  pulp  are  broken  down  and  become  a 
nauseating  mass  of  black  fungus  mycelia.  The  rind  is  left  uninjured 
until  the  disease  has  made  considerable  progress  within  but  ultimately 
a  black  and  decayed  spot  appears  on  the  surface  near  the  navel  end. 
A  certain  proportion  of  the  infected  fruits  early  shows  a  yellow  spot 
about  the  navel  end  and  drops  from  the  tree  when  the  fruits  are  one 
to  two  inches  in  diameter,  or  even  larger.  The  remainder  persist  to 
maturity,  the  disease  showing  up  at  picking  time,  in  transit,  in  storage, 
or  not  until  in  the  hands  of  the  consumer. 


JUNE  DROP   OF   WASHINGTON   NAVEL   ORANGES 


205 


During  the  spring  and  summer  of  1916  hundreds  of  cultures  were 
made  using  the  flowers  and  young  fruits  from  many  citrus  sections 
of  the  state,  from  Oroville  to  San  Diego,  and  in  a  high  percentage 
of  the  cultures  Alternaria  citri  was  obtained,  though  often  associated 
with  Macrosporium  and  some  other  fungi.  Later  in  the  season  when 
the  fruits  were  one  inch  to  two  and  one-half  inches  in  diameter  more 
cultures  were  made,  using  only  the  fruits  which  were  on  the  point 
of  dropping.      Alternaria  was  found  in  almost  all  of  these  fruits. 


Fig.  1. — Typical  Alternaria  drops.      The  discolored  area  under  the  navel  is  char- 
acteristic of  the  larger  fruits. 


Examination  of  these  larger  fruits  showed  a  dark  area  just  under 
the  navel  end  which  contained  gum.  Cultures  from  these  dark  areas 
usually  resulted  in  Alternaria.  From  work  done  in  the  spring  and 
summer  of  1916  it  appeared  evident  that  the  June  drop,  to  some 
extent  at  least,  was  due  to  the  same  fungus  parasite  causing  Navel 
end  rot;  and  that  those  fruits  which  manifested  the  rot  on  maturity, 
instead  of  measuring  the  extent  of  infection  with  Alternaria,  really 
represented  the  few  fruits  only  which  were  able  to  withstand  the 
infection,  the  larger  proportion  having  been  shed.  From  these  results 
the  theory  was  advanced  that  excessive  transpiration  from  the  leaves 
caused  water  together  with  enzymatic  poisons  secreted  by  the  fungus 


206  UNIVERSITY    OF    CALIFORNIA EXPERIMENT    STATION 

to  be  drawn  back  through  the  vascular  system  of  the  young  fruits 
through  the  pedicel,  causing  the  fruit  to  drop.2     (See  Fig.  3.) 

In  an  attempt  to  determine  the  distribution  of  Alternaria,  a  large 
number  of  open-air  exposures  with  culture  plates  was  made  in  various 
parts  of  the  state,  from  Oroville  to  Whittier.  In  practically  every 
case  Altemaria  citri  was  among  the  fungi  obtained  which  indicated 
that  the  spores  are  almost  ever-present.  Examination  during  winter 
and  spring  of  mummied  oranges  and  other  debris  under  the  trees 
disclosed  the  fact  that  the  fungus  overwinters  on  much  material.  In 
spite  of  the  prevalence  of  the  spores  the  amount  of  infection  was  very 
much  greater  in  1916  than  in  1917,  but  this  is  perhaps  accounted  for 
by  the  fact  that  the  season  of  1916  was  very  cool,  being  entirely  with- 
out hot,  dry  north  winds,  while  in  1917  the  bloom  was  five  weeks  late 
and  the  weather  subsequently  was  remarkable  for  its  long-continued 
high  temperatures. 

In  1917,  however,  after  making  cultures  of  the  very  young  fruits, 
it  was  found  that  a  great  many  were  not  infected  with  Alternaria. 
The  proportion  of  infected  fruits  to  the  total  number  shed  increased 
as  the  season  progressed.  Therefore  it  was  quite  evident  that  while 
Alternaria  was  probably  the  cause  of  much  of  the  late  dropping, 
it  could  not  be  alone  responsible  for  all  of  the  early  drop.  A  survey 
of  the  literature  on  the  abscission  of  fruits  and  other  plant  parts 
brought  out  the  fact  that  abnormal  water  relations  in  plant  tissues 
due  to  peculiar  or  unfavorable  environmental  conditions  are  fre- 
quently the  cause  of  shedding  in  a  number  of  other  plants.  Therefore 
a  careful  study  of  the  climatic  and  soil  conditions  in  the  locality  under 
consideration  was  undertaken. 


CLIMATIC    CONDITIONS    AND    THE    SHEDDING    OF    THE    YOUNG    FRUITS 

It  has  long  been  noted  that  the  yield  per  tree  of  Washington  Navel 
oranges  is  much  less  in  the  interior  valleys  than  in  the  coast  regions, 
and  the  decrease  is  greater  the  more  harsh  the  climatic  conditions 
encountered.  There  seems  to  be  a  marked  correlation  between  the 
atmospheric  temperature  and  humidity  and  the  yield,  the  lower  the 
average  humidity  and  the  higher  the  average  daily  temperature  dur- 
ing the  growing  period,  the  smaller  the  yield.  As  far  as  yield  is 
concerned  there  is  little  doubt  but  that  the  Washington  Navel  variety 
is  not  well  adapted  to  the  intense  dry  heat  of  the  interior  valleys. 
The' tree  thrives  and  produces  a  high  quality  of  fruit  where  it  has 

2  See  article  in  Univ.  Calif.  Jour,  of  Agr.,  Sept.,  1916,  "The  Cause  of  June 
Drop  of  Washington  Navel  Oranges/'  J.  Eliot  Coit  and  E.  W.  Hodgson. 


JUNE  DROP   OF   WASHINGTON   NAVEL   ORANGES  207 

water,  but  the  yield  is  greatly  reduced.  Careful  surveys  were  made 
to  obtain  data  as  to  the  relative  yield  in  different  districts.  Placing 
the  yield  of  the  most  climatically  favored  district  in  the  state  at  100 
per  cent,  the  least  favorable  locality  produced  only  25  per  cent  of  a 
crop  where  other  conditions  of  production  were  fairly  comparable. 

This  reduction  in  yield  is  not  strange  when  it  is  remembered  that 
the  citrus  tree  is  undoubtedly  a  mesophyte  of  tropical  origin  and 
as  such  is  not  well  adapted  to  withstanding  intense  heat  and  especially 
the  extremely  dry  air  of  the  interior  valleys. 


WATER    RELATIONS    AND    THE    SHEDDING    OF    IMMATURE    PLANT    PARTS 

All  the  more  recent  fundamental  plant  physiological  work  has 
indicated  that,  for  the  temperate  zones  at  least,  the  water  relation  is 
the  limiting  factor  in  plant  growth  in  the  open. 

Under  normal  conditions  the  plant  absorbs  sufficient  water  from 
the  soil  to  supply  its  needs,  make  up  for  that  lost  by  transpiration, 
and  maintain  its  water  content  approximately  constant.  Plants  well 
adapted  to  arid  or  desert  regions  where  the  air  is  very  hot  and  dry 
are  often  provided  with  various  special  structures  for  resisting  too 
great  water  loss. 

However,  plants  not  so  well  equipped  for  reducing  evaporation, 
when  grown  under  arid  conditions  frequently  lose  more  water  by 
transpiration  than  the  roots  can  absorb  in  the  same  time  from  the 
soil.  The  tissues  are  dried  out  and  a  water  deficit  occurs.  If  such 
conditions  are  prolonged  wilting  and  ultimately  death  from  dessica- 
tion  occurs.  But,  as  is  sometimes  the  case,  the  plant  can  absorb 
sufficient  water  to  supply  itself  during  the  night  and  the  greater  part 
of  the  day  when  climatic  conditions  are  not  at  a  maximum  of  severity. 
There  is  then  a  period  during  the  middle  of  the  day  when  there  is 
a  water  deficit  but  later  in  the  afternoon  and  during  the  night  this 
loss  is  made  up  and  the  plant'  recovers.  This  is  what  happens  on 
certain  days  in  the  cotton  fields  of  the  Imperial  Valley  where  during 
midday  the  leaves  actually  appear  wilted  but  are  fresh  and  turgid  the 
next  morning. 

Under  such  a  periodic  daily  fluctuation  in  water  content  many 
plants  shed  certain  parts,  such  as  flowers,  leaves,  or  immature  fruits. 
The  shed  portions  are  still  alive  and  this  process  of  abscission,  as  it  is 
called,  must  not  be  confused  with  the  shedding  of  dead  plant  parts, 
such  as  certain  autumn  leaves,  which  is  known  as  exfoliation.  Plants 
which  shed  immature  fruits  under  such  conditions  possess  a  delicate 
mechanism  which  exists  in  the  pedicel  between  the  fruits  and  the  shoot 


208  UNIVERSITY    OF    CALIFORNIA EXPERIMENT    STATION 

to  which  they  are  attached.  This  mechanism  is  not  possessed  by  all 
plants,  but  where  it  occurs  it  may  be  brought  into  action  and  the 
abscission  process  started  by  certain  stimuli  which  vary  for  different 
plants.  In  some  plants  variations  in  moisture  content  or  sudden 
fluctuations  in  temperature  are  causative  stimuli.  In  others  mechan- 
ical injury,  such  as  insect  punctures  or  chemical  injuries  due  to  fungus 
attacks,  are  sufficient  to  cause  the  shedding.  In  some,  as  in  the  case 
in  cotton,  either  will  cause  abscission. 

With  these  facts  in  mind  a  comprehensive  series  of  experiments 
was  carried  on  to  ascertain  whether  pronounced  water  deficits  occur 
in  citrus  trees  and  at  what  hours  of  the  day.  Suffice  it  to  say  here 
that  extended  investigation  has  shown  beyond  any  doubt  that  such 
do  occur  regularly  day  after  day  in  the  fruits  and  foliage  of  the 
Washington  Navel  orange  tree  during  the  hot,  dry  growing  season. 
During  the  day  the  fruits  decrease  in  water  content  as  much  as  25 
to  30  per  cent.  It  has  been  definitely  established  that  under  severe 
conditions  when  the  atmospheric  pull  is  high  the  leaves  actually  draw 
water  back  out  of  the  young  fruits  to  maintain  themselves.  But  this 
supply  is  not  sufficient  and  they  decrease  in  moisture  content  also. 
The  combined  effect  of  this  tremendous  loss  from  leaves  and  fruits 
results  in  tensions  in  the  water-conducting  system  of  the  tree.  These 
tensions  as  well  as  the  water  deficits  have  been  found  to  be  at  their 
maxima  when  environmental  conditions  are  most  severe,  that  is,  be- 
tween 10  a.m.  and  3  p.m. 

Meteorological  records  show  that  the  atmospheric  humidity  of  the 
interior  valleys  is  quite  low  during  the  growing  months,  relative 
humidities  of  15  per  cent  being  not  uncommon.  Such  humidities 
may  and  do  occur  without  marked  increase  in  air  temperature.  In 
other  words,  it  is  possible  for  extremely  dry  weather  to  occur  without 
the  characteristic  hot  norther. 

Experiments  have  been  performed  in  the  laboratories  at  Berkeley 
in  which  this  process  of  abscission  of  leaves  on  cut  branches  has  been 
induced  by  artificial  means.  The  process  itself  has  been  studied  and 
found  to  consist  in  the  gelatinization  and  dissolution  of  the  cell  walls 
resulting  in  complete  separation  of  the  cells. 

Although  further  experiments  will  be  necessary  in  order  to  clear 
up  certain  points  not  yet  entirely  settled,  all  the  evidence  so  far 
obtained  seems  to  point  to  the  following  conclusions : 

The  major  part  of  the  June  drop  occurs  early  in  the  season  and 
has  to  do  with  blossoms  and  small  fruits.  It  is  caused  by  a  stimulus 
to  abscission  arising  from  abnormal  water  relations  within  the  plant 
due  to  the  peculiar  climatic  conditions.      The  major  part  of  the  drop 


JUNE   DROP   OF   WASHINGTON   NAVEL   ORANGES 


209 


of  the  larger  oranges  is  caused  by  the  fungus  Alternaria  citri.  These 
two  causes  overlap  to  a  considerable  extent,  for  many  of  the  small 
fruits  which  succumb  to  water  relations  are  also  infected  with  Alter- 
naria. The  relative  importance  of  the  Alternaria  drop  would  not  be 
very  great  were  it  not  for  the  fact  that  it  continues  after  the  water- 
relations  drop  has  already  thinned  the  fruit  much  more  than  is 
desired. 


Fig.  2. — Not  all  these  oranges  are  infected  with  Alternaria. 

as  a  result  of  certain  climatic  conditions. 


Most  of  them  fell 


Further  evidence  that  the  cause  as  indicated  is  substantially  cor- 
rect lies  in  the  fact  that  in  certain  orchards  which  are  provided  with 
efficient  windbreaks  and  interplanted  with  alfalfa  and  heavily  irri- 
gated, the  water  deficits  in  leaves  and  fruits  have  been  found  to  be 
much  reduced  Such  orchards  have  less  drop  and  are  notable  for 
their  comparatively  large  yields.  The  Kellogg  orchard  at  Bakersfield 
is  planted  to  alfalfa  and  is  shielded  by  a  fairly  efficient  windbreak. 
Meteorological  measurements  made  in  this  orchard  and  on  the  desert 
to  windward  show  that  the  climatic  complex  is  greatly  ameliorated. 
This  of  course  is  the  result  of  a  number  of  factors  working  together. 


210  UNIVERSITY    OF    CALIFORNIA EXPERIMENT    STATION 

The  alfalfa  transpires  at  a  tremendous  rate  and  literally  bathes  the 
trees  in  a  moist  atmosphere.  The  windbreak  retards  the  movement 
of  this  relatively  moist  air  away  from  the  vicinity.  The  vaporization 
of  water  from  soil  and  plants  tends  to  lower  the  temperature  of  the 
air.  As  the  soil  is  largely  shaded,  the  high  soil  temperatures  are 
reduced,  which  temperatures  operate  to  cut  down  root  absorption  at 
the  time  of  day  when  water  loss  from  the  leaves  is  greatest. 

It  has  been  shown  by  other  investigators  that  changes  in  soil 
temperature  have  a  marked  effect  on  the  rate  of  water  intake  by  the 
roots  of  plants.  Above  and  below  certain  temperatures  (which  vary 
for  different  kinds  of  plants)  the  rate  of  absorption  by  the  roots  is 
markedly  reduced.  It  thus  seems  probable  that  under  the  prevalent 
practice  of  clean  cultivation,  during  the  middle  of  the  day  when 
transpiration  is  greatest  the  root  absorption  is  actually  reduced, 
resulting  in  an  increase  in  the  water  deficits  in  all  parts  of  the  tree. 

Not  only  are  clean  cultivated  orchards  subjected  to  higher  soil 
temperatures,  but  inasmuch  as  the  root  system  tends  constantly 
toward  the  surface  layers,  it  is  much  reduced  by  the  annual  spring 
plowing  which  shears  off  many  of  the  fibrous  feeders,  thus  reducing 
the  root  area  just  before  blooming  and  at  the  very  time  the  trees 
are  under  the  greatest  strain. 

PREVENTION    AND    CONTROL 

When  we  come  to  consider  the  prevention  or  control  of  June  drop 
we  find  that  there  are  two  lines  along  which  we  may  work.  In  the 
first  place,  it  is  entirely  possible  for  many  growers,  who  are  so  sit- 
uated as  to  make  it  practicable,  to  modify  environmental  conditions, 
both  above  and  below  ground,  *  sufficiently  to  reduce  the  extent  and 
amount  of  the  water  deficits  referred  to.  In  selecting  a  site  for  a 
navel  orchard  it  is  well  to  keep  in  mind  the  direction  of  prevailing 
winds,  the  relation  of  the  site  to  extensive  irrigated  areas  and  to  the 
degree  of  exposure  to  these  influences.  Fruit  growers  should  accus- 
tom themselves  to  think  of  climate  not  only  in  terms  of  counties,  great 
valleys  and  states,  but  also  in  terms  of  strictly  local  conditions.  A 
half  acre  of  irrigated  alfalfa  isolated  on  the  desert  will  exert  a  most 
profound  influence  on  the  climate  a  few  yards  to  leeward  and  a  few 
feet  from  the  ground.  Everyone  is  familiar  with  the  coarse,  rough 
appearance  of  oranges  grown  in  fully  exposed  positions  in  the  tops 
of  the  trees.  The  reason  why  oranges  borne  on  the  inside  of  the 
tree  are  of  smoother  texture  and  thinner  skin  is  because  the  environ- 
ment inside  the  tree  represents  quite  a  different  climate  than  that 
of  the  outside. 


.TUNE   DROP   OF   WASHINGTON   NAVEL   ORANGES 


211 


In  order  to  compare  the  evaporating  power  of  the  air  inside  and 
outside  of  an  orange  tree  at  Edison,  two  atmometer  cups  were  oper- 
ated, one  in  the  midst  of  the  tree  and  the  other  at  the  same  height 
from  the  ground  but  midway  between  two  trees.  The  readings  cov- 
ered seventeen  days  from  May  22  to  June  9,  1916,  during  which  time 
the  maximum  daily  temperatures  ran  from  80°  to  95°.  The  average 
daily  water  loss  from  the  inside  cup  was  36.8  c.c.  and  that  for  the 
outside  cup  47.9  c.c,  or  30  per  cent  greater. 

While  it  is  obviously  impossible  for  growers  to  completely  offset 
the  effects  of  hot  north  winds  such  as  occurred  June  14-17,  1917,  when 
in  addition  temperatures  between  110°  and  120°  F.  were  registered 
in  many  parts  of  the  state  south  of  the  Tehachapi,  it  is  possible  to 


Fig. 


3. — Structure  of  the  navel  orange.      The  central  pith  of 
primary  fruit  acts  as  the  stem  of  secondary  fruit. 


accomplish  a  good  deal  toward  ameliorating  such  conditions.  The 
careful  selection  of  the  site ;  the  growing  of  summer  cover  crops  such 
as  cow  peas  or  alfalfa;  adequate  irrigation;  the  reduction  of  leaf 
area  by  moderate  winter  pruning ;  closer  planting  of  the  trees  where 
possible;  the  growing  of  adequate  windbreaks;  the  mulching  of  the 
ground  to  reduce  the  soil  temperature ;  all  are  ways  to  accomplish  the 
results  desired. 

It  should  be  understood  however  that  the  authors  are  not  making 
a  general  recommendation  for  the  growing  of  summer  crops  in  citrus 
orchards,  but  merely  pointing  out  that  where  they  are  grown,  some 
reduction  in  the  amount  of  June  drop  may  be  expected.  Whether 
or  not  it  may  be  practicable  for  any  such  crops  to  be  grown  in  a 
given  orchard  must  be  decided  for  each  individual  case  separately. 

The  second  line  of  work  which  holds  some  promise  is  the  discovery 
and  propagation  of  chance  sports  or  mutations  which  may  be  satis- 


212  UNIVERSITY    OF    CALIFORNIA EXPERIMENT    STATION 

factory  in  other  ways,  yet  to  a  degree  resistant  to  the  conditions  which 
produce  the  June  drop.  Every  grower  should  watch  for  such 
mutations. 

The  proportion  of  the  shedding  caused  by  the  fungus  alone  is 
relatively  small.  Moreover,  the  mode  of  infection  is  a  serious 
hindrance  to  successful  intervention  by  means  of  fungicide  sprays. 
The  authors  have  therefore  been  led  to  conclude  that  spraying  with 
fungicides  alone  can  hardly  accomplish  enough  good  to  pay  for  the 
materials  and  labor.  Cooperative  spraying  experiments  carried  out 
during  the  last  year  have  shown  no  measurable  reduction  of  the 
June  drop. 


SUMMARY 

June  drop  of  Washington  Navel  oranges  causes  an  annual  loss  of 
1,225,000  to  $1,750,000  in  California  and  is  the  limiting  factor  in 
profitable  navel-orange  production  in  many  parts  of  the  interior 
valleys  of  California  and  Arizona.  The  drop  itself  can  be  separated 
into  two  parts,  that  occurring  from  petal  fall  until  the  fruit  is 
about  an  inch  in  diameter,  and  that  occurring  after  that  time.  The 
first  part  is  much  the  more  serious  and  is  due  to  abnormal  water 
relations  which  serve  as  a  stimulus  to  abscission.  The  second  part  is 
less  important  and  is  due  to  the  fungus  Altemaria  citri  of  wide  dis- 
tribution which  is  also  the  cause  of  black  rot  of  Navel  oranges.  The 
fruits  showing  the  black  rot  represent  those  infected  fruits  which 
managed  to  survive  to  maturity,  other  infected  fruits  having  fallen, 
chiefly  during  the  months  of  July  and  August. 

The  water-relations  drop  bears  a  definite  relation  to  climatic 
conditions  and  all  efforts  looking  toward  prevention  or  control  must 
be  either  in  the  nature  of  modifying  environmental  conditions,  or  in 
selection  for  dry-heat-resistant  strains.  Of  these  two  the  former 
seems  to  promise  the  more  immediate  results. 


